It shows the increasing event rate of PBH binaries at higher redshift. This is caused by \(P(a) \propto a^{-1/4}\)  [1]}, there are more PBH binaries having small major axes, when binaries form, which emit high frequency gravitational wave and merge at high redshift. So more high frequency gravitational wave events propagate from high redshift to the present that are redshifted and lied in frequency band of detectors. (also see in [2]}, [3]}).
